1. Field
This disclosure relates to processor devices, and particularly to asynchronous processors operating from a variable power supply voltage.
2. Description of the Related Art
In this patent, the term “processor” means a digital circuit that executes stored instructions. A processor as referred to herein may be a microprocessor, a microcontroller, a digital signal processor, a graphic processor, a coprocessor, a network processor, or some other type of processor. Most digital processors in use today are synchronous, which is to say various elements within the digital processor operate synchronously in response to a common clock signal. The power consumption of synchronous digital processor may be estimated by the formulap=0.5fcv2,
where: p=power consumption,                f=clock frequency,        c=average internal capacitance charged or discharged on each clock cycle, and        v=power supply voltage.        
For example, assume that a synchronous digital processor can be operated with a 1 GHz clock frequency at a power supply voltage of 1.0 volt. Further assume that, when operated at 1 GHz and 1 volt, the power consumption of the processor is 0.5 watt. With these assumptions, a processing task that requires 1 million clock cycles can be completed in 1 millisecond with an energy cost of 0.5 watt×1 millisecond=0.5 millijoules.
However, suppose that a time period of 1 second was actually available to complete this processing task. The task could be performed by operating the processor with a clock rate of 1 MHz. A power supply voltage of only 0.4 volts may be sufficient to run the processor with a 1 MHz clock. In this case the power dissipation of the processor (fcv^2) would drop to 0.5(1/1000)(0.4)2=0.08 mW. However the processor would run for a full second to complete the task, so the energy cost of the task would be =0.08 mW*1 second=0.08 mJ. Thus, in this example operating the processor at a lower clock speed and commensurate lower voltage reduces the energy consumption by a factor of about 6 better compared to intermittent full speed operation. In general, the energy cost of a processing task will be minimized if the task is completed over the maximum available time period at the slowest possible clock speed and power supply voltage.
Current production processor integrated circuits are synchronous and the methodology and tools for designing synchronous processor circuits are well developed, so long as the processor circuit is designed to operate from a power supply voltage substantially above the threshold voltage of the transistors comprising the processor. While research papers have shown that near-threshold operation of synchronous processors (i.e., operation with a power supply voltage near the transistor threshold voltage) may be possible, process variations and operating temperature variations can cause the delay of near-threshold circuits to vary by a factor of up to 100. Additionally, for near-threshold operation, transistor leakage currents are neither well-modeled nor well-controlled in production processes. Thus modeling delay accurately in circuits for near-threshold voltage operation is challenging, and current design tools are not suited to the design of near-threshold synchronous processors. As a consequence, near-threshold voltage operation in synchronous designs has been limited to research only.
It is possible to dynamically change the clock speed and/or power supply voltage for a synchronous processor in response to varying processing demand. However, an additional problem with synchronous processors is a need to reset/pause the processor whenever the clock frequency or voltage is changed. This results in wasted power and time that limits the benefits of dynamically changing the clock speed and/or power supply voltage.
Throughout this description, elements appearing in figures are assigned three-digit reference designators, where the most significant digit is the figure number where the element is introduced and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator.